Java jni tutorial windows

The JNI divides object references for jobject used by the native code into two categories: local and global references:. The above JNI program declares two native methods. Both of them create and return a java. Integer object.

In the C implementation, we need to get a class reference for java. Integer , via FindClass. We then find the method ID for the constructor of Integer , and invoke the constructor. However, we wish to cache both the class reference and method ID, to be used for repeated invocation. In the above program, we invoke FindClass to find the class reference for java. Integer , and saved it in a global static variable. Nonetheless, in the next invocation, this reference is no longer valid and not NULL.

This is because FindClass returns a local reference, which is invalidated once the method exits. To overcome the problem, we need to create a global reference from the local reference returned by FindClass.

We can then free the local reference. The revised code is as follows:. Take note that jmethodID and jfieldID are not jobject , and cannot create global reference. Introduction At times, it is necessary to use native non-Java codes e. JNI is difficult, as it involves two languages and runtimes. I shall assume that you are familiar with: Java. The C function simply prints the message "Hello world! A bit compiler can run on bit or bit backward compatible Windows, but bit compiler can only run on bit Windows.

A bit compiler could produce target of bit or bit. If the target is native Windows, the code can be distributed and run under Windows. However, if the target is Cygwin, to distribute, you need to distribute Cygwin runtime environment cygwin1.

This is because Cygwin is a Unix emulator under Windows. The above explains for the many versions of GCC under Cygwin. In this case " jni. Next, use the following command to compile HelloJNI. Some people suggested to use -Wl,--kill-at. JNI in Package For production, all Java classes shall be kept in proper packages, instead of the default no-name package. The outputs are: make HelloJNI. The outputs are: make all javah -classpath.. You shall see the output "Hello World!

Java Reference Types: jobject for java. It also defines the following sub-type s: jclass for java. Java array is a reference type with eight primitive array and one Object array. Hence, there are eight array of primitives jintArray , jbyteArray , jshortArray , jlongArray , jfloatArray , jdoubleArray , jcharArray and jbooleanArray ; and one object array jobjectArray. For reference JNI type, convert or copy the arguments to local native types, e.

Primitive JNI types such as jint and jdouble do not need conversion and can be operated directly. Perform its operations, in local native type. Create the returned object in JNI type, and copy the result into the returned object.

JNI is a C interface, which is not object-oriented. It does not really pass the objects. It then performs its intended operations - displays the string received and prompts user for another string to be returned. String object from an array of characters in modified UTF-8 encoding. String object from an array of Unicode characters. There are 8 sets of the above functions, one for each of the eight Java primitives.

The native program is required to: Receive the incoming JNI array e. Perform its intended operations. Convert the return C's native array e. You need to provide the variable name and its field descriptor or signature. For arrays, include a prefix "[" , e. The issue is that SetWindowsHookEx takes parameters including a pointer to the hook procedure and a handle to the DLL that contains the hook procedure.

Windows will be using that info to invoke the hook procedure. As you might imagine, Windows is not going to be able call directly into a method of an object in the JVM. That dll will contain the actual hook process, and will invoke SetWindowsHookEx for you. The dll's hook process can then use JNI to invoke your java object. Stack Overflow for Teams — Collaborate and share knowledge with a private group.

Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. Asked 9 years ago. Active 6 years, 8 months ago. Viewed 4k times. If you do not understand these technologies, you may want to bone up on them, but it is not necessary to work through this tutorial and it may not be necessary for implementing your own JNI.

I'm not too interested in Windows and will mostly focus on a Linux implementation here. However, much applies to Windows which is fully supported by the JDK in this regard. There are many other resources on the Internet such as the one you're reading. Most are as narrow and useful as mine such as the many concentrating only on the Windows DLL form. Besides whatever file editing and source management software you like to use, all you need to develop a JNI on Linux is.

For assembly, you use that file to guide your own definitions. Include jni. In addition to this convention, there are ways for the native code to access data passed from the JVM as well as to pass data back.

The env pointer is to a structure containing passed-in JVM information including function pointers by which to interact with the latter and access Java objects. Your friend in accessing objects as well as furnishing data back is jni. There are many additional types, practices and pitfalls, most of which I do not cover here, but some of which you will see in my code: jobject , jstring , etc. You will need to study my examples, plus read what you find on Google that concerns you. The purpose of this document is not to show some magnificent and meaningful JNI, but expose the basic framework.

Therefore, we're not necessarily going to do anything that would be best done from JNI, just something that illustrates that we can do something in C. The important bits to take away from perusing this document are really how to structure a JNI, access the resources needed by its implementation and how to build it. In addition, we'll illustrate accessing it from Java code in an Eclipse project.

While I'll show Windows DllMain. From the resulting class file SystemCall. It's correspondingly delicate, therefore. Any name or interface changes may break the compilation and even identifier naming. Note in particular how the String argument passed in Java is handled: it's not directly used, but is retrieved in a special way and must also be explicitly discarded in order not to leak resources.